1. Field of the Invention
The present invention relates generally to bearings and, more particularly, is concerned with a hydrodynamic pivoted pad bearing assembly for a reactor coolant pump.
2. Description of the Prior Art
In pressurized water nuclear power plants, a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump.
The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example 550 degrees F. and 2250 psi. The pump basically includes three general sections from top to bottom--motor, shaft seal and hydraulic sections. The upper motor section includes a motor which is coupled to drive a vertical pump shaft. The middle shaft seal section includes three tandem sealing assemblies located concentric to, and near the top end of, the pump shaft. Their combined purpose is to mechanically contain the high positive pressure coolant of the reactor coolant system from leakage along the pump shaft to the containment atmosphere during normal operating condition.
The lower hydraulic section includes an impeller mounted on the lower end of a pump shaft which is operable within-the pump casing to pump reactor coolant about the respective loop. The hydraulic section also includes a heat exchanger and a lower radial journal bearing assembly disposed in tandem relationship about the pump shaft above the impeller. The heat exchanger functions to reduce the rate of heat flow from the reactor coolant into the pump bearing and seal areas. The journal bearing assembly includes a two-piece horizontally split housing, a bearing cartridge, and a journal. The inside diameter of the housing is machined to a spherical shape which mates with a spherical surface on the outside diameter of the bearing cartridge to allow the bearing assembly to self-align. The bearing cartridge operates against the journal which is shrunk fit on the pump shaft. This bearing assembly is lubricated and cooled by seal injection water flow downward from the seal section to the thermal barrier area of the heat exchanger.
The lower radial bearing assembly of the abovedescribed construction has undergone many hours of operation and overall has performed satisfactorily. However, one significant drawback of the radial bearing assembly is that it tends to whirl and generate undesirable vibration due to such whirl. This is particularly evident in some pump designs as bearing radial load decreases (as the loop temperature increases). Also, over zealous pump balancing which removes all or most of the running speed component of vibration can result in a non-linearly related increase in vibration at bearing whirl frequency. Consequently, a need exist for an alternative approach to bearing design with the objective of overcoming the above-cited drawback without introducing a set of new ones.